Partner Messaging is GOOD, features available depend on the Release (R1, R6, or R7 - they skipped R2 through R5 to keep the Messaging release current with ACS Release availability). Log into the System Administrator's mailbox, but then press 7 instead of 9, and you will hear the release announced.
If you don't have any extensions assigned to a group, calling the group extension number will result in a busy signal (cause there's nothing to ring) - normal operation.
Voicemail coverage for groups is weird. The Partner Messaging manual will tell you to go into the ACS programming, and set the Transfer Return Extension for each Voice Mail Port to Ext. 10. This is so that a call transferred from the Automated Attendant to an extension that does not have Voice Mail Coverage will first ring the extension that is transferred to, and when the Transfer Return Rings expires, the call will then transfer to Ext. 10, the typical Operator in the system. BUT, this makes calls transferred from the Automated Attendant to a Group ring forever, they never time out, they never go to Ext. 10 as a last resort.
Most installers I run into DON'T set those Transfer Return Extension settings for the Voice Mail Ports - guess they missed that step in the instructions. But if you don't do that, either by accident or on purpose, a call transferred from the Automated Attendant to a Group will ring the number of times in the Transfer Return Rings setting, and then return to the Voice Mail Port that did the transfer originally. Now the Voice Mail Port answers the transfer, but there aren't any "Mode Codes" generated by the ACS to tell the Voice Mail Port what to do with the call, so the Voice Mail says "Transferring to Operator" and sends the call to the Call Answer Service Operator, typically ext. 10.
So, you want a selector code to transfer to Calling Group 2, which is Ext. 72. Partner Messaging has Transfer Restrictions turned on by default, so you have to make a "mailbox" for Ext. 72, but give it Transfer Only permissions. If you create an actual Mailbox for 72, and set it WITH a phone, the Auto Attendant will make the transfer, the call will ring the group, when it doesn't get answered it comes back to the Voice Mail Port that did the transfer, but this time it is able to recognize that there is a Mailbox that takes messages associated with Ext. 72, and that is what answers.
Now you have messages in Mailbox 72, how do you know to check them? 2 ways. Go into Mailbox 72 and set the "Phantom Extension" to Mailbox 10. The greeting in Mailbox 72 plays to the caller, but the message is left in Mailbox 10 just as if the caller reached Mailbox 10. OR, if you want to keep the incoming messages separate from Mailbox 10, make 72's Phantom Destination be Mailbox 9992, or 9993, or 9994. Make Ext. 10 the General Mailbox Owner for Automated Attendant 2, or 3, or 4. This will light up the light on Ext. 10 for both messages in Mailbox 10 AND Mailbox 9992/3/4, and the user can check both mailboxes until the light goes off, and they know which messages are for them personally, and which are from the group.
Finally, you might be able top ignore everything I've just said. On the Executive's phone, enable Call Coverage to the Assistant's extenison. Set the Call Coverage Rings as needed. A call rings at the Executive's phone for X number of rings, then rings at the Assistant's phone. If it is not answered, it goes to the Executive's mailbox. It doesn't have to be programmed on a button at the Executive's phone, although the light next to the button reminds you that the feature is enabled, and you can toggle it on and off. You can just go to the Executive's phone, press Feature-20-Executive's Extension Number-Assistant's Extension Number to enable. Feature 20-Executive's Extension Number TWICE to cancel.